1. Field of the Invention
The present invention relates to an apparatus for electric power conversion in electric rolling stock, wherein the apparatus comprises an inverter and drives an electric motor of electric rolling stock according to an alternating-current power output from the inverter.
2. Description of the Related Art
The apparatus for electric power conversion in electric rolling stock, wherein the apparatus comprises an inverter and drives an alternating-current electric motor (AC motor) according to an alternating-current power output from the inverter. For example, when the electric power is supplied through the an alternating-current catenary (AC catenary), the apparatus for electric power conversion in electric rolling stock inputs a single-phase alternating current through the AC catenary and converts it to direct-current power (DC power). At this time, the apparatus for electric power conversion in electric rolling stock requires a large-capacitance filter capacitor to reduce a direct-current voltage ripple (DC voltage ripple). On the other hand, when the electric power is supplied through a direct-current catenary (DC catenary), the apparatus for electric power conversion in electric rolling stock requires large-capacitance filter capacitors in order to reduce a high frequency component of catenary current. With regard to the both types of filter capacitors, the required capacitance depends on the output of an applied device. Especially, the apparatus for electric power conversion in a high-power electric rolling stock such as a locomotive has filter capacitors with especially large capacitance.
On the other hand, the apparatus for electric power conversion in electric rolling stock is required to reduce surge voltage of a switching element (semiconductor element), which constitutes an electric power conversion section, at interrupting. There are two methods for reducing the surge voltage.
One method is a method in which a snubber circuit is provided in the electric power conversion section. A smaller apparatus for electric power conversion in electric rolling stock has been required in order to be installed in electric rolling stock. Thereby, the apparatus for electric power conversion in electric rolling stock preferably has a configuration without a snubber circuit in order to decrease the number of components by making the circuit of the electric power conversion section simple, and to minimize extra power consumption for better efficiency.
Thus, the second method is proposed in which a filter capacitor is connected to the electric power conversion section. The filter capacitor connected between DC terminals forms a loop circuit with a switching element included in the electric power conversion section. In this method, it is desired to connect the filter capacitor by “low inductance connection” in which the loop circuit has as low an inductance as possible. In order to realize the low inductance connection, an element which has a large inductance, such as a fuse, cannot be connected to the loop circuit.
There has been disclosed an apparatus for electric power conversion (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-96832) provided with an electric power conversion section in which a large-capacitance filter capacitor and the electric power conversion section are connected by the low inductance connection, and a snubber circuit is not provided.
However, a measure regarding damages of the apparatus for electric power conversion has not been disclosed in the above document, wherein the damages are caused by a direct-current short-circuit current generated when breakdown of a switching element in the electric power conversion section is caused.
For example, when a short-circuit is made in a switching element within the electric power conversion section by a fault, charges in the filter capacitors discharge at a fault location. At this time, degree of breakdown in the switching element becomes large, depending on electrical discharge energy. Accordingly, there is a possibility that not only a portion of the switching element is subjected to damages caused by the above fault, but also an outside housing in the electric power conversion section bursts, and peripheral devices are subjected to damages.
There has been a method by which a fuse is provided between the filter capacitor and the switching element in order to reduce flow of electrical discharge energy into the fault location. Thereby, the fuse melts while discharging to reduce flow of the electrical discharge energy into the fault location. However, the filter capacitor and the switching element cannot be connected by the low inductance connection in this method.
Accordingly, these filter capacitors preferably has as low an capacitance as possible from the viewpoint of reducing damages in the event of a fault. However, considering an original function of decreasing the DC voltage ripple, these filter capacitors are required to have a capacitance equal to or larger than a predetermined value according to applied devices and the like.